Conventionally, an electron tube which is used for detecting the presence or absence of a flame on the basis of ultraviolet rays emitted from the flame in a combustion furnace or the like has been known. The electron tube includes a sealed container which is sealed and filled with predetermined gas, an electrode supporting pin that penetrates through the sealed container, and two electrodes that are supported in parallel with each other by the electrode supporting pin within the sealed container. In the electron tube, when one electrode arranged to oppose the flame is irradiated with ultraviolet rays in a state where a predetermined voltage is applied across the electrodes through the electrode supporting pin, electrons are emitted from the one electrode due to the photoelectric effect and excited in succession one after another to cause an electron avalanche between the one electrode and the other electrode. Therefore, it is possible to detect the presence or absence of a flame by measuring a change in impedance between electrodes, a change in voltage between electrodes, and electric current flowing between electrodes. Various methods for detecting the presence or absence of a flame have been suggested.
In the related art, there has been suggested a method in which electric current flowing between electrodes is integrated and it is determined that a flame is present in a case where an integrated value is greater than or equal to a predetermined threshold value and a flame is absent in a case where the integrated value is less than the predetermined threshold value (for example, see PTL 1).
The invention of PTL 2 has an object to provide a flame detecting device capable of reliably detecting a flame to be detected at all times regardless of a change in ambient light such as sunlight. In PTL 2, the flame detecting device detects illuminance of ambient light such as sunlight and automatically adjusts detection sensitivity of ultraviolet rays emitted by a flame in accordance with the detected illuminance such that the flame is reliably detected regardless of a change in ambient light. The flame detecting device copes with a change in a surrounding environment.